1. Field of the Invention
The present invention relates to a method of manufacturing an arc tube to be used as a light source such as a headlamp for vehicles.
2. Description of the Related Art
In recent years, an arc tube has often been used as a light source such as a headlamp for vehicles because it can carry out high-intensity irradiation.
As shown in FIG. 5, an arc tube to be used as the headlamp for vehicles generally comprises an arc tube body 114 made of quartz glass in which pinch seal portions 114b1 and 114b2 are provided on both sides of a light emitting tube portion 114a forming a discharge space 112, and a pair of electrode assemblies 116A and 116B pinch-sealed with the pinch seal portions 114b1 and 114b2 such that tip portions thereof are protruded toward the discharge space 112.
The arc tube is manufactured by forming the light emitting tube portion 114a on a quartz glass tube and sequentially forming the pinch seal portions 114b1 and 114b2 on both sides thereof. At each pinch-sealing step, a portion to be pinch-sealed is pinch-sealed with a pincher immediately after it is heated with a burner. Consequently, the pinch seal portions 114b1 and 114b2 are formed.
As shown in FIGS. 6A and 6B, it is necessary to fill the discharge space 112 with a liquefied inert gas at a second pinch-sealing step (that is, a step of forming the second pinch seal portion 114b2 on a quartz glass tube 114xe2x80x2 provided with the light emitting tube portion 114a and the first pinch seal portion 114b1). Therefore, the light emitting tube portion 114a is cooled. As shown in FIG. 6A, the cooling operation has conventionally been carried out by jetting liquid nitrogen from a cooling nozzle 104 provided on the side of the light emitting tube portion 114a toward the light emitting tube portion 114a in a state in which a thermal insulating plate 102 is provided in a boundary position between the light emitting tube portion 114a and the portion 114b2xe2x80x2 to be pinch-sealed in the outer peripheral space of the quartz glass tube 114xe2x80x2 provided almost vertically with the first pinch seal portion 114b1 provided in a lower part.
In the conventional manufacturing method, however, the cooling nozzle 104 is provided laterally in the vicinity of the lower part of the thermal insulating plate 102. Therefore, the liquid nitrogen is vaporized in the early stage through heat transfer from the thermal insulating plate 102 and a burner 106. Consequently, the light emitting tube portion 114a is cooled insufficiently. Depending on the circumstances, the liquefied inert gas filled in the discharge space 112 is vaporized (expanded), and the light emitting tube portion 114a bursts immediately after the pinch-sealing operation using a pincher 108 so that the inert gas leaks as shown in FIG. 6B.
The present invention has been made in consideration of the above-mentioned circumstances and has an object to provide a method of manufacturing an arc tube which can increase the cooling efficiency of the light emitting tube portion through the jet of the liquid nitrogen, thereby preventing the light emitting tube portion from bursting at the second pinch-sealing step.
The object of the present invention is achieved by devising the arrangement of the cooling nozzle.
The present invention provides a method of manufacturing an arc tube including pinch seal portions on both sides of a light emitting tube portion in which a second pinch seal portion is formed on a quartz glass tube provided with the light emitting tube portion and a first pinch seal portion, the method comprising the steps of:
providing the quartz glass tube almost vertically with the first pinch seal portion positioned in a lower part and providing a thermal insulating plate in a boundary position between the light emitting tube portion and a portion to be pinch-sealed in an outer peripheral space of the quartz glass tube;
jetting liquid nitrogen from a cooling nozzle provided obliquely below the light emitting tube portion toward the light emitting tube portion, thereby cooling the light emitting tube portion in this state, and heating the portion to be pinch-sealed with a burner; and
pinch-sealing the portion to be pinch-sealed with a pincher immediately thereafter.
If the xe2x80x9ccooling nozzlexe2x80x9d is provided obliquely below the light emitting tube portion and serves to jet the liquid nitrogen toward the light emitting tube portion, a specific structure including the number of the cooling nozzles to be provided and a liquid nitrogen jet angle is not restricted particularly.
With the above-mentioned structure, in the method of manufacturing an arc tube according to the present invention, liquid nitrogen is jetted from the cooling nozzle provided obliquely below the light emitting tube portion toward the light emitting tube portion in order to cool the light emitting tube portion at the second pinch-sealing step. Therefore, the cooling nozzle is set somewhat apart from the thermal insulating plate. In that case, moreover, the thermal insulating plate as well as the light emitting tube portion can be cooled through the jet of the liquid nitrogen to be carried out obliquely upward. Consequently, the cooling nozzle is not easily influenced by heat transferred from the thermal insulating plate and the burner. For this reason, the liquid nitrogen jetted from the cooling nozzle is vaporized slowly so that the light emitting tube portion is fully cooled. Accordingly, the liquefied inert gas filled in the discharge space is not vaporized (expanded) for a while after the pinch-sealing operation. Consequently, the light emitting tube portion can be prevented from bursting to leak the inert gas.
According to the present invention, thus, the cooling efficiency of the light emitting tube portion which is obtained through the jet of the liquid nitrogen can be increased at the second pinch-sealing step. Consequently, it is possible to prevent the light emitting tube portion from bursting to leak the inert gas.
In the present invention, furthermore, the liquid nitrogen is jetted obliquely upward. Therefore, the thermal insulating plate as well as the light emitting tube portion can be cooled. Consequently, it is possible to prevent the thermal insulating plate itself from being deteriorated.
According to the present invention, moreover, the burner and the cooling nozzle are positioned sufficiently apart from each other. Therefore, it is possible to prevent the burner from being cooled through the cooling nozzle to reduce a thermal efficiency thereof.
With the above-mentioned structure, if a portion of the thermal insulating plate in the vicinity of the outer periphery of a quartz glass tube is formed like an upward taper, a cooling space formed around the light emitting tube portion can be reduced and the liquid nitrogen can be prevented from being scattered in such a direction as not to contribute to the cooling operation of the light emitting tube portion. Consequently, the cooling efficiency can be enhanced still more.
While the specific structure of the cooling nozzle is not particularly restricted as described above, it is preferable, for the following reasons, that the liquid nitrogen jet angle is set upwardby 10xc2x0 to 60xc2x0 with respect to a horizontal plane.
More specifically, the cooling nozzle is heated through heat transfer from the thermal insulating plate at an angle of less than 10xc2x0 so that the light emitting tube portion is often cooled insufficiently. On the other hand, if the angle exceeds 60xc2x0, the liquid nitrogen is jetted upward from a clearance between the thermal insulating plate and the quartz glass tube. Consequently, the heating temperature of the lower end of the portion to be pinch-sealed is dropped so that insufficient melting operation is often carried out. The angle is not set to 10 to 60xc2x0 but preferably 20xc2x0 to 50xc2x0, and more preferably 30 to 45xc2x0.
With the above-mentioned structure, it is preferable, for the following reasons, that the inside diameter of the tip portion of the cooling nozzle should be set to 3 mm or less and the distance between the tip surface of the cooling nozzle and the outer surface of the light emitting tube portion should be set to 40 mm or less.
When the inside diameter of the tip portion of the cooling nozzle exceeds 3 mm, a large amount of liquid nitrogen is thus jetted in an unnecessary direction other than the light emitting tube portion. Consequently, the liquid nitrogen is wasted. In the case in which the inside diameter of the tip portion of the cooling nozzle is set to 3 mm or less and the distance between the tip surface of the cooling nozzle and the outer surface of the light emitting tube portion exceeds 40 mm, a portion of the quartz glass tube against which the liquid nitrogen hits is greatly changed due to a fluctuation in the discharge pressure of a gas cylinder for supplying the liquid nitrogen. Consequently, the light emitting tube portion is often cooled insufficiently.
With the above-mentioned structure, in the case in which the liquid nitrogen is repeatedly jetted through the cooling nozzle in a plurality of stations, it is preferable that the liquid nitrogen jet angle of the cooling nozzle in each of the stations should be set to have an almost equal value in order to cool the light emitting tube portion efficiently.
With the above-mentioned structure, in the case in which heating operation is repeatedly carried out through the burner in a plurality of stations, it is preferable, for the following reasons, that the heating power of the burner in each of the stations should be set to be gradually increased every movement to a new one of the stations.
That is, it is preferable that the heating power of the burner should be maximized immediately before the pinch-sealing operation in order to carry out the pinch-sealing operation reliably. If the heating power is increased from the beginning, the quartz glass tube is unnecessarily melted and the cooling efficiency of the light emitting tube portion is reduced.